Fix errouneus reset of ss->threatMove

After we set ss->threatMove we could go under a IID step that
resets SearchStack ss and so also ss->threatMove.

When later we use that field in futility pruning we have this
set to MOVE_NONE !

The fix is to use a local variable and add threatMove to SplitPoint
to pass this move to slaves.

Spotted by Ralph Stoesser, fix suggested by Richard Vida.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>